The objectives of this research are to characterize the prosthetic group of the "hemoprotein" myeloperoxidase, to then synthesize this heme and compare its properties to the native system in order to understand the mechanism of action of myeloperoxidase. The synthesis of vitamin B12 biosynthetic precursors Factors I, II and III is planned along with siroheme. The study of these model isobacteriochlorins will be made in order to explain the mechanisms of action of the sulfite and nitrite reductases. Models for vitamin B12 will be synthesized and their chemistry, particularly that of the cobalt-carbon bond, will be studied in order to mimic and explain the mechanism of action of the vitamin B12 coenzyme.